This invention relates to a mercury removal method and system for use in wet gas purification. More particularly, it relates to a mercury removal method which can effectively remove very small amounts of mercury components present in a gas during wet gas purification such as coal or heavy oil gasification gas purification and petroleum refining.
Exhaust gas from coal-fired thermal electric power plants contains mercury originating from coal. This mercury cannot be completely removed in a conventional flue gas treatment system (including an electrostatic precipitator, a wet flue gas desulfurizer and the like), and some of it is discharged therefrom. Since mercury is a trace component and has a very high vapor pressure and, in particular, metallic mercury has the property of being insoluble in water, it is difficult to remove mercury by recovering it with a dust collector or by washing the gas with a scrubber.
A large amount of mercury is discharged from conventional waste disposal by incineration or the like, but the scale of disposal is relatively small and produces a small volume of gas. Accordingly, such mercury has frequently been treated, for example, by adsorption using activated carbon. While treating methods involving adsorption by activated carbon are effective methods for the removal of such mercury, they are not suitable for practical use in the treatment of a large volume of gas because an enormous consumption cost is required.
Consequently, a mercury removal method has been proposed in which an oxidizing agent is sprayed, for example, in a mist eliminator (M/E) installed downstream of a flue gas desulfurizer. Since it is difficult to use activated carbon in thermal electric power plants for the above-described reason, this method provides a more convenient means for removing mercury by spraying an oxidizing agent in a mist eliminator.
Moreover, a process has been proposed in which metallic Hg is oxidized to HgCl2 on a catalyst such as a denitration catalyst and this HgCl2 is removed in a flue gas desulfurizer.
Mercury exists chiefly in two forms: metallic mercury (Hg) of zero valence and mercury chloride (HgCl2). While metallic mercury is hardly soluble in water, mercury chloride is relatively soluble in water. Thus, mercury in the form of mercury chloride can be removed by means of a desulfurizer. Accordingly, metallic mercury of zero valence can be removed by oxidizing it to mercury chloride with the aid of an oxidizing agent.
In this process, therefore, a chlorinating agent such as Cl2 or HCl is added and sprayed just before a denitration catalyst within a denitrator, so that metallic mercury is oxidized on the denitration catalyst.
In ordinary exhaust gases, all mercury is not present in the form of metallic mercury. A certain proportion thereof is present in the form of mercury chloride because coal has a high chlorine content, and this mercury component can be removed. Accordingly, a chlorinating agent may be used for the remaining metallic mercury.
However, examination of the mercury contained, for example, in coal or heavy oil gasification gas has revealed that almost all mercury is present as metallic mercury under a reducing atmosphere and little is dissolved in water. Accordingly, if an oxidizing agent is sprayed under an atmosphere of a reducing gas during wet gas purification, the oxidizing agent will be wasted owing to the presence of various reducing substances and cannot be expected to produce any beneficial effect.
Moreover, if a chlorinating agent is continuously sprayed to induce a reaction on the catalyst, a gasification gas having a high ammonia content and a high pressure undergoes the reaction of ammonia with HCl resulting in the precipitation of ammonium chloride (NH4Cl). This ammonium chloride may cause a problem in that it is likely to accumulate in such units as GGHs and block them up.
In view of the above-described problems, the present inventors made intensive investigations in order to develop a mercury removal method which can remove mercury, as a trace component in gases, effectively and efficiently, which can reduce the mercury removal cost resulting from the operation of the system, and which requires a simplified procedure and system and can hence be carried out easily.
As a result, the present inventors have now found that the coexistence of H2S in a gasification gas causes metallic mercury to pass into water and that the so-collected is released into the gaseous phase when the water is exposed to a lower pressure (or flashed). That is, in the case of wet gas purification, the coexistence of hydrogen sulfide in the water washing step permits Hg to pass into the absorbing fluid and be removed thereby, and the Hg captured in the water washing step can be released into the gaseous phase by returning the Hg-containing waste water from the elevated pressure to atmospheric pressure. Thus, it has also been found that the above-described problems can be solved by removing mercury according to a method utilizing such phenomena. The present invention has been completed from this point of view.
Specifically, the present invention provides a mercury removal method for the removal of mercury components present in a gas during wet gas purification, the method comprising a water washing step for bringing a gas containing mercury components into contact with an absorbing fluid under pressurized conditions including the presence of not less than 10 ppm and preferably not less than 100 ppm of hydrogen sulfide so as to cause mercury components to pass from the gas into the absorbing fluid; a flashing step subsequent to the water washing step, for spraying the discharged absorbing fluid under a lower pressure to separate it into gaseous components and waste water; and an adsorption removal step for passing the gaseous components through a mercury remover provided with an adsorbent to remove mercury components therefrom by adsorption. In this mercury removal method, it is preferable to dissolve mercury components in the absorbing fluid, for example, under an elevated pressure of 0.2 to 5.0 MPa and in the coexistence of about 500 ppm to 10% of hydrogen sulfide, and remove the flashed mercury components by adsorption to activated carbon used as the adsorbent. Preferably, the activated carbon has an S component deposited thereon.
The present invention also provides a mercury removal system for the removal of mercury present in a gas during wet gas purification, the system comprising a water washing tower in which a gas containing both mercury components and hydrogen sulfide is introduced thereinto and an absorbing tower is circulated through the tower under pressurized conditions so as to cause mercury components to pass into the absorbing fluid; a flash drum in which the absorbing fluid discharged from the water washing tower is sprayed under a lower pressure to separate it into gaseous components and waste water; and a mercury remover provided with an adsorbent in which the mercury components present in the gaseous components are removed by adsorption. Typically, the aforesaid water washing tower comprises a gas cooling tower and a gas cleaning tower. In this system having a flash drum and a mercury remover installed on the downstream side of the water washing tower, about 50 to 95% of the mercury present in the formed gas introduced into the system can be removed.
The present invention also provides the above-described system that further comprises a hydrogen sulfide absorption tower in which the water-washed gas fed from the aforesaid water washing tower introduced thereinto and an absorbing fluid containing an amine compound is used to remove hydrogen sulfide by absorption into the absorbing fluid; a second flash drum in which the absorbing fluid discharged from the hydrogen sulfide absorption tower is sprayed under a lower pressure to separate it into gaseous components and an absorbing fluid to be fed to a regeneration tower; and a mercury remover provided with an adsorbent in which the mercury components present in the gaseous components delivered from the second flash drum are removed by adsorption. In this system having a flash drum and a mercury remover installed on the downstream side of the hydrogen sulfide absorption tower, about 50 to 95% of the mercury present in the water-washed gas introduced into the hydrogen sulfide absorption tower can be removed.
In the present invention, Hg can be removed by the coexistence of hydrogen sulfide in the water washing tower of the system. That is, if the gas being treated is a system involving the coexistence of hydrogen sulfide, Hg passes into the water present in the water washing tower and can hence be removed from the gas. As a result, Hg is contained in waste water discharged from the water washing tower.
When the collected Hg-containing waste water is returned from the elevated pressure to atmospheric pressure, Hg is released into the gaseous phase. Specifically, Hg is dispersed into the gaseous phase by passing the waste water through a flash drum.
Since hydrogen sulfide is usually present in a gasification gas subjected to gas purification, Hg present in the gas passes into an absorbing fluid within a water washing tower. After this absorbing fluid is passed through a flash drum to recover gaseous components, Hg can be adsorbed and captured by passing the gaseous components through an adsorbent. The present invention comprises a system in which Hg-containing gaseous components separated by flashing is passed through an adsorbent to remove mercury therefrom by means of the adsorbent. Thus, as compared with the case in which the whole gasification gas is directly treated with an adsorbent prior to its introduction into the system, Hg can be removed by treating a much smaller volume of gas, and the operating cost required for treatment with an adsorbent can be markedly reduced.
Thus, the present invention makes it possible to remove mercury, as a trace component in gases, effectively and efficiently and also to reduce the mercury removal cost resulting from the operation of the system.